This invention relates to image and unwanted signal rejection in portable dual mode receiver circuits. The invention has particular application to cellular telephone technology wherein a single compact battery powered receiver must be able to operate on at least two disparate frequency bands with different channel spacing. In particular, the invention relates to problems associated with conversion and rejection of signals for digital cellular telephones operative in the AMP/PCS/GSM/EDGE bands (850 MHz, 900 MHz, 1900 MHz bands) which have 200 kHz channel spacing and in the IS136 band (˜850 MHz band) which has 30 kHz channel spacing.
There is a need to minimize avoidable components to save power and space in critical applications such as miniature portable cellular receiver circuit devices. The challenge is to provide rejection of unwanted signals and images at the input into the digital subsystem.
Many signal conversion systems with passive low-pass filters of sufficiently steep rolloff to attenuate adjacent and next-adjacent channels or images are difficult to realize. The spacing of the channels or signals and images is such that energy is often well within the skirts of a typical rolloff characteristic of a passive low-pass filter.
What is therefore needed is a filter circuit which provides adequate signal rejection at frequencies of interest and yet which is simple to realize and construct in the context of mass manufacturing.
Active twin-T filters and passive twin-T filters are known. However, their use in combination or as alternatives in the same circuit is not.